The present invention relates to testing of video equipment, and more particularly to three-dimensional testing of video codecs to accurately characterize the performance of the codecs.
Television pictures are digitally transmitted over a wide range of channel capacities. Transmitting coders and receiving decoders, commonly referred to as "codecs", are available for many different applications with different picture quality requirements. Broadcast quality codecs commonly use channels with a capacity of 45 Mbits/sec, which are called DS3 links by common carriers, while codecs for video teleconferencing may use channels with data rates of 1.5 Mbits/sec (T1 data links) or even lower. Since a 525 line, 30 frame/sec NTSC television picture requires almost 100 Mbits/sec and the corresponding picture in component format requires over 200 Mbits/sec, data rate reduction or compression factors ranging from 2:1 up to more than 100:1 are used.
Conventional video test signals have not proven adequate for testing the performance of low data rate codecs. The compression algorithms used in these codecs remove redundancy in the picture. Conventional video test signals, such as color bars, linearity, pulse and bar, and multiburst, have no line-to-line variations, and each frame is typically identical. These test signals are vertically and temporally correlated. Therefore the total amount of information in the test signal is much less than the information in a typical television program picture. The compression algorithms perform better on such test signals than on typical program pictures so that more complex test signals are required to more accurately characterize the codecs' performance.
Differential pulse code modulation (DPCM) is commonly used to achieve modest compression ratios. The simplest form of DPCM takes advantage of picture redundancy in a single direction. The codec estimates the current pixel to be the same as its neighbor, taken either horizontally, vertically or temporally. Some codecs use a weighted three-dimensional estimate.
Transform coding together with coefficient thresholding also is used to achieve high quality at low data rates. Transform coding may be combined with motion estimation to take even greater advantage of program picture redundancy.
Codecs commonly include buffers to deal with picture complexity variations. Test signals that are locally complex but globally simple may not fill the buffer. If the test signal does not fill the buffer, the test signal does not test the picture degradations due to channel capacity.
What is desired is a video test signal that provides test pictures with varying amounts of spatial and temporal correlation so that picture distortion may be measured as a function of picture complexity.